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Food Irradiation and Chlorine Team Up to Kill E. coli O157:H7 and Salmonella

By Doris Stanley Lowe
June 1, 1999

Treating alfalfa seeds and sprouts with a combination of irradiation and chlorine effectively safeguards them against contamination by E. coli O157:H7 and Salmonella, Agricultural Research Service scientists report.

ARS scientists Donald W. Thayer, Kathleen T. Rajkowski and William F. Fett found that a treatment of irradiation and chlorine solution not only killed both organisms, but extended the shelf life of sprouts from about five days to more than a week. They conducted the lab studies at the ARS Eastern Regional Research Center’s Food Safety and Plant Science and Technology Research Units in Wyndmoor, Pennsylvania. The research is part of the effort by a task force of representatives from several Federal Government agencies and industry to find ways to control microbial contamination of sprouts .

The finding is good news for sprout growers. Since 1995, raw alfalfa sprouts have been recognized as a source of foodborne illness in the United States, with several outbreaks of both E. coli O157:H7 and Salmonella. FDA and the Centers for Disease Control and Prevention have advised those at high risk—including children, the elderly, and persons with compromised immune systems—to avoid eating raw alfalfa sprouts. Since sprouts can’t withstand abrasive physical washing because of their fragility, cleaning the seed has become the primary focus.

In the tests, the scientists used the irradiation dose approved for irradiating meat. The U.S. Food and Drug Administration (FDA) has approved ionizing radiation as a safe and effective food preservation tool.

Along with irradiation, they subjected alfalfa seeds to 2-percent, 2.5-percent, and 3-percent weight-per-volume concentrations of calcium hypochlorite (a chlorine source). A 3-percent concentration equals about 20,000 parts per million of available chlorine. With a neutral pH of about 7, the 2.5 and 3-percent concentrations reduced E. coli O157:H7 99.99 percent. The pH level is important because at a higher pH level, such as 10, the chlorine would change to a form that would not be as effective in killing bacteria.

According to the scientists, the best way to eliminate pathogens would be a combination of irradiation and sanitation treatments. This is because sprouts can be contaminated internally, which would prevent a surface disinfectant from working effectively.

ARS is USDA’s chief scientific research agency. More information on this story appears in the agency’s June Agricultural Research magazine, available at:


Scientific contact: Donald W. Thayer, ARS Food Safety Research Unit, Eastern Regional Research Center, Wyndmoor, PA 19038, phone (215) 233-6582, fax (215) 233-6406,